Trade has been a major source of income to civilizations throughout history. Journeying to distant lands, caravans exchanged their loads of goods for money or other items of value. In addition, the traders who accompanied the caravans often brought back new knowledge about the advances made by the civilizations with whom they did business. Marco Polo, for example, went with traders to China. He remained there for many years, and brought back much valuable information to the West.